In a conventional injection molding apparatus, melt material is delivered from an injection molding machine and flows through a hot runner manifold generally having a plurality of circular cross-section manifold channels. The manifold has an inlet and a plurality of outlets communicating with a plurality of hot runner nozzles and is heated to maintain the melt at a consistent and flowable temperature. Several variables affect the quality of the molded parts produced using hot runner manifolds. One such variable is the shear stress-induced flow imbalance that can be observed or measured along the melt channels and at the outlets of the manifold. This flow imbalance is unavoidable and is characterized by a variable, non-symmetrical, cross-sectional distribution (or profile) of the temperature, viscosity and velocity of the melt along each melt channel of the manifold. Therefore, the temperature, viscosity and velocity cross-sectional distribution or profile of the melt leaving the manifold varies at the entrance of each nozzle. This explains why in many applications the molded parts differ from one cavity to the other and from one batch to the other in terms of weight, density, size, appearance, etc.
As the melt material flows in generally circular channels, the material in the center of the channel has a higher velocity than the material along the sides of the channel. Since material along the sides of the channel moves more slowly than material in the center, it is exposed to the heat from the manifold for a longer period of time than the faster, more centrally-disposed material causing a temperature imbalance between material in the middle of a channel and material along the sides of a channel. At the same time, melt material against the sides of a channel are further heated and stressed (i.e., sheared) by the friction generated as the melt moves against the side channels. Higher temperatures and shear stress create changes in viscosity of the material.
FIG. 1 shows, in cross-section, a conventional two-level hot runner manifold 112 of a multi-channel injection molding apparatus. Melt material enters the manifold along a channel 102. The melt is maintained at a moldable temperature by manifold heaters 128. The melt then splits and enters identical and opposite branches 103 and flows around a first approximately 90-degree turn 104. The melt then splits again and enters identical and opposite branches 105 and 106, curves around a second approximately 90-degree turn 107 in each branch and exits the manifold through outlets 108 and 109, respectively. The outlets 108 and 109 are in fluid communication with two hot runner nozzles (not shown) to deliver the melt to either a single or two mold cavity system (not shown).
The shear stress created along the walls of channel 102 is schematically shown in FIG. 1A in a cross-sectional shear profile along line A-A. When channel 102 splits into branches or melt channels 103, the shear stress from melt channel 102 is greater along side 103a than on side 103b of melt channels 103. As melt material flows through branch 103, shear stress is naturally created at a lesser extent along a side 103b. However, any shear stress formed by friction along side 103a is added to the shear history from channel 102 along side 103a, forming a side-to-side, or asymmetrical, shear stress profile imbalance within channel 103. Further, shear stress profile imbalance occurs as melt moves around turns 104 and further flows along melt channel 103. Where channel 103 splits into branches or melt channels 105 and 106, the shear stress and thus the temperature and velocity profile along and across these melt channels and towards outlets 108 and 109 becomes even more unevenly distributed and unevenly balanced. The variations of shear stress profile from side 105a to side 105b are shown schematically in a cross-sectional shear stress profile along line B-B, shown in FIG. 1B. Shear stress profile variations from side 106a to side 106b are generally shown in a cross-sectional shear profile along line C-C, shown in FIG. 1C. Thus, at manifold outlets 108 and 109 each of the cross-sectional shear stress profiles of FIGS. 1B and 1C indicate distinct side-to-side variations and thus uneven shear stress, temperature and viscosity cross-sectional distribution with respect to a central axis 115 of the manifold melt channels.
Further, comparison of cross-sectional shear profiles of FIGS. 1B and 1C indicates that the amount of shear stress between manifold channels 105 and 106 differs greatly. Since shear stress profiles are also an indication of temperature, velocity and viscosity profiles, the melt that leaves branch 105 through outlet 108 has a much higher temperature on the outer and intermediate portion of the melt than the melt material that leaves channel 106 through outlet 109. Thus, a temperature and pressure suitable for molding a product from the melt in branch 105 may be different from a temperature and pressure suitable for molding a product from the melt material in branch 106. Since it is very difficult to adjust or locally correct the temperature and pressure differences to a particular channel in a multi-channel injection molding apparatus, variations in shear stress profiles lead to inconsistent molded products from one mold cavity to another and from one batch of molded products to the next.
Further, side-to-side (or uneven or non-symmetrical) shear stress and temperature cross-sectional profiles may cause different flow characteristics from one side to the other of a single molded product, causing poor quality parts to be produced.
Attempts have been made to either reduce, eliminate, redistribute or rotate the non-symmetrical profile of the temperature, viscosity or velocity of the melt flowing inside a manifold towards several nozzles in order to provide at the manifold outlets more homogeneous, identical or similar profiles that would improve the processing conditions. However, these attempts generally require splitting the melt stream via a mechanical obstruction, which may lead to flow lines, particularly with materials that are sensitive to the development of flow lines.
Reference is made in this regard to European Patent Publication No. EP 0293756, U.S. Pat. No. 5,421,715 and U.S. Pat. No. 6,572,361 that show so-called manifold melt mixers, U.S. Pat. No. 5,683,731 that shows one so-called manifold melt redistributor and U.S. Pat. No. 6,077,470 that shows a so-called melt flipper, or melt rotating device. Further, reference is also made to U.S. Patent Application Publication No. 2004/0130062 that shows yet another melt mixing device and method. Each of these references is incorporated by reference herein in its entirety, respectively.
There is a need to provide a melt redistribution device and method that will provide a melt flow through a hot runner system with an improved temperature, viscosity, pressure and shear stress cross-sectional profile at various stages of the melt flow through the system.